First-order phase transition in UO 2 : 235 U and 17 O NMR study
First-order phase transition in UO 2 : 235 U and 17 O NMR study
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UO 2 中的一级相变：235 U 和 17 O NMR 研究
DOI：
10.1103/physrevb.63.104404
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发表时间：
2001
影响因子：
3.7
通讯作者：
Y. Ōnuki
中科院分区：
文献类型：
--
作者：
K. Ikushima;S. Tsutsui;Y. Haga;H. Yasuoka;R. Walstedt;N. Masaki;A. Nakamura;S. Nasu;Y. Ōnuki
本文报道了具有反铁磁有序的5-f-Jahn-Teller(JT)体系的核磁共振研究，其中我们在5 f-电子体系中进行了有磁性(235U)和非磁性配体(17O)的核磁共振研究。在一级转变温度TN=30.8K以下，235U超精细相互作用符合轴对称的5f波函数特征，从而通过磁有序和JT扭曲相结合的方式解除了立方晶场中的轨道简并.另一方面，17O自旋-回波衰变中的调制现象提供了恰好在T_N以下的晶格扭曲的证据，这在氧位引起轴对称的电场梯度。这些结果表明，在5f四极之间的耦合和晶格畸变的协同JT相变中，可以用铀核磁共振来研究5f四极的行为，而用非磁性配体核磁共振来研究晶格的畸变。在根据中子散射实验和理论工作提出的磁结构和JT畸变中，非共线的3k型结构是最有可能的，因为1k和2k都会引起正交变形。对于低能自旋波激发，两个位置的核自旋晶格驰豫速率T1−1都在TN以下呈现T7行为，这表明由于强烈的磁振子-声子耦合而存在无间隙激发.
A nuclear magnetic resonance (NMR) study is reported for a 5 f-cooperative Jahn-Teller (JT) system with antiferromagnetic ordering, UO 2, where we have performed magnetic actinide (235 U) as well as nonmagnetic ligand (17 O) NMR studies in a 5 f-electron system. The observed 235 U hyperfine interaction is consistent with an axially symmetric 5 f-wave-function character for U 4+ below a first-order transition temperature T N= 30.8 K. Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in the 17 O spin-echo decay provide evidence of a lattice distortion just below T N, which gives rise to an axially symmetric electric-field gradient at the oxygen sites. These results indicate that, in a cooperative JT transition which occurs as a result of coupling between 5 f quadrupoles and lattice distortion, the behavior of the 5 f quadrupoles can be investigated using uranium NMR, while the lattice distortion can be studied with the nonmagnetic ligand NMR. Among magnetic structures and JT distortions, proposed on the basis of neutron scattering experiments and theoretical work, the noncollinear 3 k-type structure is the most likely, because either the 1 k or 2 k would cause an orthorhombic distortion. With respect to low-energy spin-wave excitations, nuclear spin-lattice relaxation rates T 1− 1 at both sites show a T 7 behavior below T N, which suggests the presence of gapless excitations due to strong magnon-phonon coupling.